+1 to making sure you take the time to warm up properly. If it's arthritis, a certain amount of movement keeps the joint healthier than if the horse is kept still. Yumove equine is a brilliant joint supplement (at least, it has some very positive results in dogs). Do you have access to a physiotherapist?
You have to be careful with the ground of the arenas in which you train, take time to walk so that your horse warms up well and the joints are ready to work when you start trotting etc.
You can give your horse supplement like linseed oil, glucosamine and biotin, it will help!